This invention relates to the utilization of bubbles of air in two size ranges, one fine, the other coarse (defined below), to accomplish different functions in the operation of a vertical skein of hollow fiber membranes for filtration of an aqueous medium from a biomass; and to a method of forming a header for potting the ends of the fibers in the vertical skein so as to collect permeate.
The term "vertical skein" in the title (hereafter "skein" for brevity), specifically refers to an integrated combination of structural elements including (i) a multiplicity of vertical fibers of substantially equal length; (ii) a pair of headers in each of which are potted the opposed terminal portions of the fibers so as to leave their ends open; and, (iii) permeate collection means held peripherally in fluid-tight engagement with each header so as to collect permeate from the ends of the fibers.
The term "fibers" is used for brevity, to refer to "hollow fiber membranes" of porous or semipermeable material in the form of a capillary tube or hollow fiber. The term "substrate" refers to a multicomponent liquid feed. A "multicomponent liquid feed" in this art refers, for example, to fruit juices to be clarified or concentrated; wastewater or water containing particulate matter; proteinaceous liquid dairy products such as cheese whey, and the like. The term "particulate matter" is used to refer to micron-sized (from 1 to about 44 .mu.m) and sub-micron sized (from about 0.1 .mu.m to 1 .mu.m) filtrable matter which includes not only particulate inorganic matter, but also dead and live biologically active micro-organisms, colloidal dispersions, solutions of large organic molecules such as fulvic acid and humic acid, and oil emulsions.
The term "header" is used to specify a solid body in which one of the terminal end portions of each one of a multiplicity of fibers in the skein, is sealingly secured to preclude substrate from contaminating the permeate in the lumens of the fibers. Typically, a header is a continuous, generally rectangular parallelpiped (prism), or, a cylindrical disk of arbitrary dimensions formed from a natural or synthetic resinous material.
Except for their opposed ends being potted, there is no physical restraint on the fibers of a skein. The term "skein fibers" is used herein to refer to plural arrays. An "array" refers to plural, essentially vertical fibers of substantially equal lengths, the one ends of each of which fibers are closely spaced-apart, either linearly in the transverse (y-axis herein) direction to provide at least one row, and typically plural rows of equidistantly spaced apart fibers. Less preferably, a multiplicity of fibers may be spaced in a random pattern. Typically, plural arrays are potted in a header and enter its face in a generally horizontal x-y plane (see FIG. 3). The width of a rectangular parallelpiped header is measured along the x-axis, and is the relatively shorter dimension of the rectangular upper surface of the header; and, the header's length, which is its relatively longer dimension, is measured along the y-axis.
The parent patent and application teach how to use bubbles to maintain clean fiber surfaces during microfiltration (MF) or ultrafiltration (UF) with a skein. The problem was that coarse bubbles which are most effective for scrubbing fibers are inefficient and uneconomical to provide an oxygen-containing gas (typically air, or air enriched with oxygen, occasionally pure oxygen) required for growth of microorganisms in the aqueous substrate. When furnished as fine bubbles, oxygen is dissolved in the substrate with an efficiency up to 10 times as much as when it is furnished as coarse bubbles. Particularly in large systems for the microfiltration of liquids, the combination of a foraminous conversion baffle and means to channel fine bubbles under the conversion baffle above which the skein is disposed, provides a surprisingly effective and economical solution.
In a vertical skein, all fibers in the plural rows of fibers, rise generally vertically while fixedly held near their opposed terminal portions in a pair of opposed, substantially identical headers to form the skein of substantially parallel, vertical fibers. Typically the opposed ends of a multiplicity of fibers are potted in closely-spaced-apart profusion and bound by potting resin, assuring a fluid-tight circumferential seal around each fiber in the header and presenting a peripheral boundary around the outermost peripheries of the outermost fibers. The position of one fiber relative to another in a skein is not critical, so long as all fibers are substantially codirectional through one face of each header, open ends of the fibers emerge from the opposed other face of each header, and substantially no terminal end portions of fibers are in fiber-to-fiber contact. The vertical skein is not the subject matter of this invention and any prior art vertical skein may be used. Further details relating to the construction and deployment of a most preferred skein are found in the parent U.S. Pat. No. 5.639,373, and in Ser. No. 08/690,045, the relevant disclosures of each of which are included by reference thereto as if fully set forth herein.
For MF at about ambient pressure, a skein securing at least 10, preferably from 50 to 50,000 fibers, each generally at least 0.5 m long, is disposed within a reservoir of substrate, above the conversion baffle.
The fibers divide a reservoir into a "feed zone" and a withdrawal zone referred to as a "permeate zone". The feed (substrate) is introduced externally (referred to as "outside-in" flow) of the fibers, and resolved into "permeate" and "concentrate" streams. The skein, or a bank of skeins is most preferably used for either MF or UF with "outside-in" flow.
The unique method of forming a header disclosed herein allows one to position a large number of fibers, in closely-spaced apart relationship, randomly relative to one another, or, in a chosen geometric pattern, within each header. It is preferred to position the fibers in arrays before they are potted to ensure that the fibers are spaced apart from each other precisely, and, to avoid wasting space on the face of a header; it is essential, for greatest reliability, that the fibers not be contiguous. By sequentially potting the terminal portions of fibers in stages as described herein, the fibers may be cut to length in an array, either after, or prior to being potted. The use of a razor-sharp knife, or scissors, or other cutting means to do so, does not decrease the open cross-sectional area of the fibers' bores ("lumens"). The solid potting resin forms a circumferential seal around the exterior terminal portions of each of the fibers, open ends of which protrude through the permeate-discharging face of each header, referred to as the "aft" face.
The system disclosed herein is most preferably used to oxygenate mixed liquor in activated sludge, such as is generated in the bioremediation of wastewater. An oxygen-containing gas serves the dual purpose of providing fine bubbles for living microbes as well as coarse bubbles for scrubbing the fibers. Surface portions of the fibers are contacted by successive bubbles as they rise, whether the air is supplied continuously or intermittently, and the fibers are kept "awash in bubbles."